Random access memory (RAM) is a ubiquitous component of modern digital architectures. RAM can be a standalone device, or can be integrated in a device that uses the RAM, such as a microprocessor, microcontroller, application specific integrated circuit (ASIC), system-on-chip (SoC), and other like devices. RAM can be volatile or non-volatile. Volatile RAM loses its stored information whenever power is removed. Non-volatile RAM can maintain its memory contents even when power is removed. Although non-volatile RAM has advantages, such as an ability to retain its contents without applied power, conventional non-volatile RAM has slower read/write times than volatile RAM.
Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology having response (read/write) times comparable to volatile memory. MRAM data does not degrade over time and, compared to other PAM technologies. MRAM uses very little power. In contrast to conventional RAM technologies, which store data as electric charges or current flows, MRAM uses magnetic storage elements, such as a magnetic free layer.
Magnetic tunnel junctions (MTJ's) comprise two layers of magnetic material separated by a thin insulator. The magnetization direction of one magnetic layer is fixed and the magnetization direction of the other magnetic layer can be changed by applying a voltage or current to the MTJ. When the magnetization directions of the fixed and the free layers are opposite or antiparallel, the MTJ has a higher resistance than when the magnetization directions of the fixed and free layers are parallel. The resistance of an MTJ can be measured by applying a small read current thereto. The MTJ can represent a digital “0” in one of these magnetic states and to represent a digital “1” in the other state, thus allowing the MTJ to be used as a magnetic memory element in MRAM.
Due in part to fabrication variations, the resistances of MTJs may vary. Thus, it is necessary to measure the resistances of MTJs as part of the fabrication process to ensure that the MTJs meet performance standards.
FIG. 1 depicts a conventional direct analog MTJ resistance measurement technique 100 for testing an MTJ 105 that is coupled in series with an access transistor (T) 110. During testing, a constant voltage (V) is applied across the MTJ 105 and the access transistor 110 at a hit line (b1) 115 and a source line (s1) 120. The access transistor 110 is actuated via a word line (w1) 125 to permit flow of an analog test current (I) 130. The analog test current 130 is measured, and the resistance (RMTJ) of the MTJ 105 is calculated as RMTJ=(V/I)−Rt, where Rt is the on-resistance of the access transistor 110.
FIG. 2 depicts another conventional direct analog MTJ resistance measurement technique 200 for testing the MTJ 105. As sense amplifier 205 input is coupled to the bit line 115. A reference resistor (R) 210 is coupled as a second input to the sense amplifier 205. During testing, a step waveform 215 is applied to the second input of the sense amplifier 205 across the reference resistor 210. The step waveform 215 can be a stepped voltage having a step ΔVref or a stepped current having a step ΔIref. As the stepped waveform 215 is applied, the number of steps (n) are counted until the output 220 of the sense amplifier 205 changes, at which time the resistance (RMTJ) of the MTJ 105 is calculated. The Rmtj=R*n*ΔIref/ISA−Rt or Rmtj=n*ΔVref/ISA−Rt, where ISA is the current through the MTJ and ΔIref and ΔVref define the resolution of the direct analog MTJ resistance measurement technique 200.
Problems with conventional testing devices and methods include a testing time that is too long, which results in long test times and reduced foundry throughput. Further, it is difficult for external logic testers to generate an accurate multi-channel analog multi-step waveform during the conventional process.
Accordingly, there are long-felt industry needs for methods and apparatus that improve upon conventional methods and apparatus, including methods and apparatus for testing a resistive memory element.